1. Field of the Invention
This invention pertains to a vehicle brake system employing two separate air pressure supplies and an improved brake assembly compatible therewith for connecting to the two supplies and utilizing the pressures supplied thereby under varying conditions of operation.
2. Description of the Prior Art
Parking, emergency and service air brake systems that are presently employed on trucks, trailers, truck tractors, busses and other air-brake equipped vehicles generally include dual diaphragm brake chambers for all non-steerable axles and single diaphragm brake chambers on all steerable axles. The pressure supply systems which are employed in such air brake systems generally comprise compressors and reservoirs along with multiple valves employed for the various operating modes of the brake system. There are generally two separate air systems employed, namely, a service or primary system, and an emergency or secondary system. Not only are the valves numerous, but they vary in complexity of construction. Variously such valves include service relay valves, spring brake control valves, quick release valves and others. Operationally, treadle valves are commonly used for service brake applications and pressure-regulated, push-pull valves are used for parking and emergency applications. Further, the power or pressure supply equipment utilize one-way and two-way check valves or other protection valves to protect one air system from another.
The purposes of having two air systems and the rather complex valving arrangements that operate therewith are many. One purpose is to provide safety in the event that there is a complete loss of air of one of the systems or at a connection between the tractor and the truck trailer of a motor truck vehicle. A further feature provided by such systems is to allow a driver to release and reapply brakes in the event of pressure loss in one side of the system. Therefore, it is apparent that safety and back-up operation are important in air brake systems. In fact, federal and state highway safety laws require that air systems meet certain requirements, including not only having a primary/secondary pressure operation, but also with regard to providing braking for emergency and parking operation in the event of complete loss of pressure.
The most prevalent systems employed in the prior art to meet this latter requirement utilizes a "spring brake," the operation of which is relatively simple. Such a spring brake operates in conjunction with a dual diaphragm brake chamber, namely, a service diaphragm brake chamber and an emergency diaphragm brake chamber. During normal driving operations the emergency diaphragm receives air pressure from the emergency reservoir keeping the brake spring compressed. This brake spring, when not compressed, operates in a direction to actuate the brakes. As the service brakes are applied, air is applied against the service diaphragm, causing the push or actuator rod of the brakes to advance and apply the brakes.
When there is a loss of air in the service system, the service brakes become inoperable and the spring is activated by exhausting air from the emergency diaphragm, thereby permitting the brakes to be applied by the mechanical force of the spring, which is also augmented in some cases by manual effort.
A common shortcoming of such a system is evident when there is a failure of the emergency air system. When there is a loss of emergency air pressure, the air applied against the emergency diaphragm will exhaust and the brakes will be applied by the spring automatically, even at highway speeds. Such result can be unexpected and even cause loss of control of the vehicle.
Another shortcoming of the prior art system just described is in conjunction with applying the brakes when parking. When the driver of the vehicle decides to stop and park the vehicle by applying the parking brake, the service air is unaffected. The emergency air against the emergency diaphragm is exhausted to allow the spring to move the actuator rod forward to apply the brakes. The same separation occurs when a trailer is disconnected from the tractor or other towing vehicle. That is, the air supply lines, and especially the emergency air supply line, is disconnected. This vents or exhausts the emergency diaphragm and causes the spring to set the brakes. Brakes are released by restoring the air pressure against the emergency diaphragm. If it is desired to release the brakes, there must be an air supply. Otherwise, the brakes can be manually released only by employing a special tool that requires considerable effort to operate and which exposes the user to safety hazards.
In further discussion of the shortcomings of prior art system, consider also the hazardous nature of the mechanism. The spring contained in the chamber must be extremely strong to brake a heavy vehicle. It can be under as much as 1800 pounds of force. Hence, it can literally explode from its housing should the housing corrode and weaken. Further, corrosion cannot be readily inspected. Moreover, should a mechanic remove the clamp ring retaining the spring without first taking the precaution of suitably externally restraining the housing parts, the spring will explode from the housing. Such an event has often caused injury and even death.
The chambers are large in size and are heavy. Because of their proximity to the axle, there is not much shock absorbing or cushioning of the chambers. Hence, they are subject to axle vibration, which can cause loss of air through the connections of the chamber or lines and valve parts connected thereto. As mentioned, high speed lockups will occur when there is a sudden emergency line pressure loss, such as with a line rupture.
Another disadvantage of the prior art air system described above is that it can cause severe and premature brake lining and brake drum wear. This wear results from brake drag caused by seal leaks between the emergency and service chambers. That is, when air restraining the spring leaks into the service chamber, this can cause the brakes to partially apply. Hence, drum and lining contact is made, resulting in reduced fuel economy, increased brake heat that reduces application forces and increased lining and drum wear.
Therefore, it is a feature of the present invention to provide an improved air brake system generally avoiding the safety and maintenance problems associated with conventional air brake systems employing spring brakes.
It is another feature of the present invention to provide an improved air brake assembly utilizing a single diaphragm with a mechanical locking device without using a brake-actuation spring.
It is yet another feature of the present invention to provide an improved air brake assembly utilizing a simplified two-way check valve in combination with a single diaphragm assembly in such a manner to avoid sudden brake applications but assuring brake applications in emergency and parking situation when the power supply lines are disconnected intentionally or unintentionally.